The present invention relates generally to a metering system for deriving performance parameters in an A.C. electrical power system, and is more particularly concerned with a system and method for deriving real and reactive power parameters in a power system for use in revenue metering and/or overload protection applications.
In revenue metering applications it has heretofore been customary to meter real and reactive power in A.C. electrical systems by means of devices which are predominantly electro-mechanical in nature, such as induction disc type watt-hour meters, which generally provide better than 1 percent accuracy and are available with accuracies approaching 0.25 percent.
Recently, with the advent of economical microprocessor devices, it has been proposed that real and reactive power measurements be accomplished by electronic means, thereby avoiding completely the need for electromechanical mechanism and achieving a reduction in physical size and cost. Previous attempts at such electronic metering, which were based on the use of analog techniques utilizing multipliers and operational amplifiers, typically were not able to duplicate the accuracy of the electromechanical watt-hour meters, especially in the wide temperature range environments encountered in A.C. power systems.
In A.C. power system circuit protection applications, wherein system parameters, such as voltage, current, and power are continuously monitored and a relay or other switch device is opened upon occurrence of a system fault, it has heretofore been customary to use electro-mechanical relays to protect machines, capacitor banks, transmission lines, transformers, reactors, distribution and transmission busses, and other power system components. Typically, each of these electro-mechanical relays has one or more current or voltage sensors, and is arranged to protect only one circuit or device in the system, so that failure of one relay does not necessarily result in loss of protection to other system components protected by other relays.
Over the past several years electronic computer-based protection systems have been developed, thus avoiding the need for each relay to have its own parameter sensor, and thereby allowing a greater degree of operational flexibility. Unfortunately, due to the cost of the computer hardware required for monitoring line conditions using presently practiced methods, the concept of using computer-based protection systems has not been economically feasible for protection of a single component with a single relay, and has been generally limited to centralized applications wherein several system components can be protected, such as all the components in a substation. Thus, the autonomy and reliability of the conventional electro-mechanical relays is lost as centralization of protection becomes necessary to make computer-based protection and monitoring economically feasible. For this reason, computer-based protection systems have not gained wide industry acceptance.
The present invention provides a metering system which is compact and economical to construct, and which overcomes the deficiencies of prior art revenue metering and fault protection circuits by providing output signals which indicate power system parameters with a high degree of accuracy and stability, in a form compatible with digital processing techniques.
Accordingly, the general object of the present invention is to provide a new and improved system and method for deriving performance parameters such as real and reactive power in an A.C. electrical power system.
A related object of the present invention is to provide digital output signals from the above system, and to utilize these signals in revenue metering and fault protection applications.